Damping
Keeping friction to a minimum permits measuring small currents, but creates a major problem when reading the meter.
When a meter measures current, the pointer should move across the scale and stop immediately at the correct reading. However, because of the very little friction of the rotating parts, the pointer does not come to rest immediately at the correct reading; it overshoots because of inertia, and then the spring pulls it back; it overshoots slightly again, and so on. As a result, the pointer tends to swing back and forth or vibrate, about the correct reading point many times before coming to rest.
To overcome this problem, the meter movement must be damped. Damping can be thought of as a braking action on the rotating parts. It almost completely eliminates the vibrating action of the pointer, resulting in quick, correct pointer indication.
Damping also eliminates another problem. When a meter is removed from an external circuit or when the circuit is de-energized, the pointer returns to zero. Because of the very low friction of the rotating parts, the springs tend to pull the parts back to zero very quickly; so quickly in fact, that the pointer could bend as it overshoots and strikes the left retaining pin. This is particularly true when the meter returns to zero from near full -scale deflection.
Damping a Moving-Coil Meter
Moving-coil meter movements make use of the aluminum frame on which the coil is wound to provide damping. Since aluminum is a conductor, the frame acts as a one-turn coil. When the coil assembly and pointer rotate to register current, the aluminum frame cuts through the field flux lines of the permanent magnet. Small currents, called eddy currents, are induced in the frame, which set up a magnetic field about the frame.
The polarity of this magnetic field is opposite to that of the magnetic field about the coil. Therefore, the magnetic field about the frame opposes the magnetic field about the coil. This action reduces the overall field of the moving coil so that it swings more slowly. In effect, the faster the coil swings, the more the aluminum frame’s field slows it down. This causes the coil and pointer to rotate relatively slowly and smoothly to the correct reading without vibrating. As soon as the coil assembly and pointer come to rest, no further eddy currents are induced in the frame and its magnetic field disappears.
When the meter is disconnected from the circuit, or when the circuit is de-energized, essentially the same action occurs. The coil assembly and pointer start to rotate very rapidly toward zero, but now, since it is rotating in the opposite direction, the eddy currents produced in the frame flow in the opposite direction. This results in a magnetic field with a polarity opposite to the one previously. As the springs pull the coil assembly and pointer closer and closer to zero, the like poles of the permanent magnet and the frame repel each other more and more so that the coil assembly is again braked or slowed down, and slowly comes to rest at zero. Thus, the pointer is prevented from striking the left retaining pin and, perhaps, bending around it.
References
http://www.electrical-engineering-assignment.com/damping-devices
http://electriciantraining.tpub.com/14175/css/Damping-24.html
